ZTS: get_persistent_disk_name can return truncated names

Instead of using only the 3rd element return the entire string after
the split to handle device names with dashes.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Vaibhav Bhanawat <vaibhav.bhanawat@delphix.com>
Closes #15567